Method for spherizing granular polyetrafluoroethylene powder

ABSTRACT

To provide a method of shaping a granular polytetrafluoroethylene powder, in which a powder flowability can be enhanced and an apparent density can be increased without substantially changing an average particle size and particle size distribution of the powder. The granular polytetrafluoroethylene powder having an average particle size of 100 to 800 μm is subjected to shaping by using a rotating-type stirring vessel with two cross axes.

TECHNICAL FIELD

The present invention relates to a method of shaping a granularpolytetrafluoroethylene (PTFE) powder.

BACKGROUND ART

PTFE, particularly PTFE prepared by suspension polymerization is oncepulverized into an average particle size of not more than about 100 μm,and then granulated by various granulation methods such as drygranulation and wet granulation to be a granular powder having anaverage particle size of from about 100 μm to about 800 μm. The granularpowder is used as it is as a molding powder for powder molding. Howevereven if the powder is simply granulated, the granulated powder is poorin powder flowability, and therefore usually after the granulation step,a shaping step is provided for shaping of the granulated powder, therebyenhancing a powder flowability and increasing an apparent density.

For example, a granular PTFE powder obtained by granulating with aribbon mixer and then with a flash mixer is again treated with a ribbonmixer for shaping at a decreased number of rotations.

In that shaping method with a ribbon mixer, though an apparent densityand powder flowability of a granular PTFE powder after the shaping areenhanced, there are problems that granular powders are agglomerated andparticles having a large particle size are obtained, which results inshifting to a larger average particle size and increase of a largerparticle size region in particle size distribution.

Those problems also arise in case where granular PTFE powders obtainedby other granulation methods are subjected to shaping with a ribbonmixer.

An object of the present invention is to provide a method of shaping agranular PTFE powder, which enhances an apparent density and powderflowability without substantially changing an average particle size anda particle size distribution.

DISCLOSURE OF THE INVENTION

The present invention relates to the method of shaping of a granularPTFE powder, characterized in that a granular PTFE powder granulated toan average particle size of from 100 to 800 μm is put in a stirringvessel having two or more rotation axes crossing each other and issubjected to stirring for shaping by rotating the stirring vessel on itsrotation axes.

The stirring is carried out under the conditions for increasing anapparent density of the granular PTFE powder without substantiallychanging its average particle size and particle size distribution.

It is preferable that two or more rotation axes of the stirring vesselare disposed at a right angle to each other. A represented preferableexample thereof is a cross rotary mixer.

With respect to the stirring conditions of the cross rotary mixer, it ispreferable that a speed of revolution is from 5 to 20 rpm and a speed ofautorotation is from 10 to 30 rpm.

A granular PTFE powder to be subjected to shaping may be deagglomeratedbefore the shaping.

As a granular PTFE powder to be subjected to shaping, a granular powdergranulated by a dry granulation method or other granulation method canbe used.

A granular PTFE powder to be subjected to shaping may contain or doesnot contain a filler.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a partial perspective view of a cross rotary mixer used in thepresent invention for explaining directions of autorotation andrevolution.

FIG. 2 is a diagrammatic view of an apparatus used for determining aflowability in examples of the present invention.

FIG. 3 is a graph showing a particle size distribution of granular PTFEpowders subjected to shaping in Example 1 of the present invention andComparative Example 1 and a starting granular PTFE powder.

FIG. 4 is a graph showing a relation between a shaping time, apparentdensity and average particle size in Example 4 of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A feature of the present invention is to use a stirring vessel havingtwo or more axes, particularly two cross axes for shaping (hereinafterreferred to as “rotating-type stirring vessel with two cross axes”).When the rotating-type stirring vessel with two cross axes is used,since the granular PTFE powder becomes round without being agglomerated,an apparent density can be increased and a powder flowability can beenhanced without changing an average particle size and a particle sizedistribution.

The shaping method of the present invention is a method for putting agranular PTFE powder obtained by various granulation methods into arotating-type stirring vessel with two cross axes and then stirring thepowder by rotating the stirring vessel on each of the two rotation axesfor a given period of time.

An example of a rotating-type stirring vessel with two cross axes whichcan be suitably used in the present invention is a cross rotary mixerwhich has been used for mixing of various powders. The cross rotarymixer is usually used for mixing of metals, ceramics, chemicals,cosmetics, foods, etc. from a sanitary point of view and from theviewpoint of its mixing performances.

Known types of cross rotary mixers are as follows.

(1) Cross rotary mixer with a high speed stirring chopper

(2) Cross rotary mixer with a baffle plate

(3) Cross rotary mixer with a binder injection nozzle

Any of those mixers can be used in the present invention.

In the cross rotary mixer used in the present invention, as shown inFIG. 1, the autorotation means the rotation of the stirring vessel 1 onthe axis r, and the revolution means the rotation on the axis R.

With respect to the relation between the speed of autorotation and thespeed of revolution of the cross rotary mixer, when the speed ofautorotation is assumed to be 1, it is preferable, but indefinite, toadjust the speed of revolution (referred to as “a ratio ofrevolution/autorotation”) to 0.1 to 2 from the point that an apparentdensity can be increased more and a powder flowability can be enhancedmore without changing an average particle size and particle sizedistribution.

A stirring time varies depending on a desired particle size and isoptionally selected experimentally. In general with the advance ofstirring, an apparent density increases but the increase in apparentdensity reaches its upper limit in a certain period of time. Thestirring time is usually from about 5 minutes to about 30 minutes.

The stirring with the cross rotary mixer is carried out under theconditions of 5 to 60 rpm of autorotation speed, 5 to 40 rpm ofrevolution speed, 0.3 to 1 of a ratio of revolution/autorotation and 5to 30 minutes of stirring time. Under those conditions, a granular PTFEpowder subjected to shaping and being excellent particularly in anapparent density and powder flowability can be obtained without changingan average particle size and particle size distribution. Furtherpreferred conditions are 10 to 30 rpm of autorotation speed, 5 to 20 rpmof revolution speed, 0.5 to 0.8 of a ratio of revolution/autorotationand 5 to 30 minutes of stirring time. Under such conditions, a granularPTFE powder subjected to shaping and being further excellent in anapparent density and powder flowability can be obtained.

From the point that shaping of a granular powder is carried outpreferably with heating, it is preferable to maintain the stirringvessel at a temperature of from about 40° C. to about 80° C., usuallyfrom about 40° C. to about 70° C. by a method of recirculating hot wateror the like method.

With respect to granular PTFE powders which can be used for the methodof shaping of the present invention, a granulation method is notlimited. Granular PTFE powders prepared by various granulation methodscan be used. Also not only a filler-containing granular PTFE powder butalso a non-filler-containing granular PTFE powder can be subjected tothe shaping of the present invention.

In general, a non-filler-containing granular PTFE powder after thegranulation has an average partial size of 100 to 800 μm and an apparentdensity of 0.4 to 0.6 g/cc. When such a granular PTFE powder issubjected to the shaping of the present invention, its average particlesize and particle size distribution do no change substantially, and itsapparent density increases to 0.7 to 1.0 g/cc and its powder flowabilityis enhanced. Also in case of a filler-containing granular PTFE powder,its average particle size and particle size distribution are the same asthose of the non-filler-containing granular PTFE powder, and a possiblerange of its apparent density is as wide as 0.7 to 1.0 g/cc depending onkind and amount of the filler. In that case, too, according to theshaping of the present invention, an apparent density and powderflowability can be increased without changing an average particle sizeand particle size distribution. In either case, for example, a change inan average particle size after the shaping is within ±10%, preferablywithin ±5%, more preferably within ±3%. Also the particle sizedistribution after the shaping does not change substantially or becomesrather narrower.

As mentioned above, a granulation method of a granular PTFE powder isnot limited, and there are, for example, the following methods. Examplesthereof are a method of stirring a fine PTFE powder in water at hightemperature (cf. JP-B-47-8372), a method of adding a fine PTFE powder toan organic solvent being capable of wetting PTFE to give a slurry andthen tumbling with a blender, etc. (cf. JP-B-43-6290, JP-B-44-22620), amethod of wetting a fine powder with a water-insoluble organic solventbeing capable of wetting PTFE and then stirring in water (cf.JP-B-44-22619), a method of wetting a PTFE powder with an aqueoussolution containing a surfactant and then tumbling with a blender, etc.(cf. JP-B-54-17782, WO97/17392), a method of wetting a PTFE powder withan aqueous solution containing a surfactant to give a slurry and thenstirring in water in the presence of an organic solvent (cf.WO97/15611), a method of stirring a PTFE powder in water in the presenceof an organic solvent being capable of wetting PTFE and a surfactant(cf. WO97/11111), and the like.

The granular PTFE powder used in the present invention is a powdergenerally called a molding powder, and is prepared by once pulverizing astarting powder obtained by suspension polymerization into a size ofseveral tens of microns to hundreds of microns and then granulating thepulverized powder.

In case of a molding powder, the smaller a particle size is, the moredense a molded article is and the better a surface condition of themolded article is. On the contrary, there are drawbacks from theviewpoint of its handling that a powder flowability is lowered, anapparent density is low and a powder is easily solidified. The granularPTFE powder of the present invention has enhanced handling propertythough properties of the molded article obtained therefrom are somewhatsacrificed.

Examples of PTFE are tetrafluoroethylene (TFE) homopolymer andcopolymers of TFE which are not melt-processable (a content of comonomeris usually up to 5% by weight).

As the monomer copolymerizable with TFE, there are, for example, aperfluoro(vinyl ether) represented by the formula (I):

CF₂═CF—OR_(f)  (I)

wherein R_(f) is a perfluoroalkyl group having 1 to 10 carbon atoms, aperfluoro(alkoxyalkyl) group having 4 to 9 carbon atoms, an organicgroup represented by the formula (II):

in which m is 0 or an integer of 1 to 4, or an organic group representedby the formula (III):

in which n is an integer of 1 to 4, and the like.

The number of carbon atoms of the above-mentioned perfluoroalkyl groupis from 1 to 10, preferably from 1 to 5. When the number of carbon atomsis within the above-mentioned range, an effect of making creepresistance excellent can be obtained with maintaining the property ofbeing not-melt-processable.

As the above-mentioned perfluoroalkyl group, there are, for example,perfluoromethyl, perfluoroethyl, perfluoropropyl, perfluorobutyl,perfluoropentyl, perfluorohexyl and the like. From the viewpoint ofcreep resistance and cost of monomer, perfluoropropyl is preferable.

When a proportion of the monomer copolymerizable with TFE is within therange of 1.0 to 0.001% by mole, creep resistance of a molded articleobtained from the granular powder can be improved.

The granular PTFE powder used in the present invention may contain afiller. Examples of the filler are, for instance, one or more of metalfiber powders or metal powders such as glass fiber powder, graphitepowder, bronze powder, gold powder, silver powder, copper powder,stainless steel powder, stainless steel fiber powder, nickel powder andnickel fiber powder; inorganic fiber powders or inorganic powders suchas molybdenum disulfide powder, fluorinated mica powder, coke powder,carbon fiber powder, boron nitride powder and carbon black; organicpowders such as heat-resistive aromatic resin powder, e.g.polyoxybenzoyl polyester, polyimide powder,tetrafluoroethylene-perfluoro(alkyl vinyl ether) copolymer (PFA) powderand polyphenylene sulfide powder; and the like. The filler is notlimited thereto.

In case where two or more fillers are used, preferable combination is,for example, glass fiber powder and graphite powder, glass fiber powderand molybdenum disulfide powder, bronze powder and molybdenum disulfidepowder, bronze powder and carbon fiber powder, graphite powder and cokepowder, graphite powder and heat-resistive aromatic resin powder, carbonfiber powder and heat-resistive aromatic resin powder or the like. Themixing method may be either of wet method or dry method.

It is preferable that an average particle size or an average fiberlength of the above-mentioned filler is from 10 to 500 μm.

With respect to the proportion of the PTFE powder and the filler, it ispreferable that the proportion of the filler is from 2.5 to 100 parts(parts by weight, hereinafter the same), more preferably 5 to 80 partson the basis of 100 parts of the PTFE powder.

The granular PTFE powder obtained by granulating the PTFE powder or thePTFE powder containing a filler by the above-mentioned variousgranulation methods can be subjected to shaping of the present inventionas it is. On the other hand, as mentioned below, the granular PTFEpowder may be once deagglomerated and then subjected to shaping with arotating-type stirring vessel with two cross axes such as a cross rotarymixer. With the deagglomeration step, there is an advantage that theparticle size can be made fine.

As the deagglomeration methods, there are a method of deagglomeratingwith a flash mill (JP-B-44-22620), a method of deagglomerating with ahomomixer (JP-B-44-22619), and the like. The deagglomeration method isselected depending on the method of granulation.

Deagglomeration conditions vary depending on a deagglomerating machineto be used , a desired particle size, etc. From the viewpoint of makinga particle size proper, it is preferable to select the deagglomerationconditions where an average particle size after the deagglomeration isfrom 100 to 800 μm, preferably from 300 to 700 μm. In the method ofgranulating in water, from the viewpoint of productivity, it ispreferable that the deagglomeration is carried out, for example, byexternal circulation from a granulation tank by using a homomixer.

According to the shaping method of the present invention, an apparentdensity can be increased and a powder flowability can be enhancedwithout substantially changing an average particle size and particlesize distribution of the granular PTFE powder after the shaping step.

EXAMPLE

The shaping method of the present invention is then explained by meansof examples, but the present invention is not limited to them.

In the present invention, physical properties were measured by thefollowing methods. Apparent density:

Measured according to JIS K 6891-5.3.

Flowability 1

Measured in accordance with the method described in JP-A-3-259925.

Namely, there is used a measuring apparatus comprising a support base42, an upper hopper 31 and a lower hopper 32. The both hoppers arealigned on their center lines and supported on the support base 42 asshown in FIG. 2 (corresponding to FIG. 3 described in JP-A-3-259925).The upper hopper 31 has an inlet 33 of 74 mm diameter, an outlet 34 of12 mm diameter and a partition plate 35. The height from the inlet 33 tothe outlet 34 is 123 mm. The partition plate 35 is provided on theoutlet 34, and thereby the powder in the hopper can be kept therein anddropped optionally. The lower hopper 32 has an inlet 36 of 76 mmdiameter, an outlet 37 of 12 mm diameter and a partition plate 38. Theheight from the inlet 36 to the outlet 37 is 120 mm, and the partitionplate 38 is provided on the outlet 37 like the upper hopper. The upperhopper and the lower hopper are adjusted so that the distance betweenthe both partition plates is 15 cm. In FIG. 2, numerals 39 and 40represent outlet covers of each hopper, and numeral 41 represents avessel for receiving the dropped powder.

Prior to measuring the flowability, about 200 g of powder is allowed tostand for not less than four hours in a room, the temperature of whichis adjusted to 23.5° to 24.5° C., and then sieved with a 10 mesh sieve(sieve opening: 1,680 μm). The measurement of the flowability is carriedout at the same temperature.

(I) At first, immediately after the upper hopper 31 is charged with justa cup of powder by using a 30 cc cup, the partition plate 35 is pulledout to drop the powder into the lower hopper. When the powder does notdrop, the powder is stuck with a wire. After the powder has droppedcompletely into the lower hopper 32, the dropped powder is allowed tostand for 15±2 seconds, and then the partition plate 38 of the lowerhopper is pulled out to see whether or not the powder is dropped fromthe outlet 37. When the powder is dropped completely within eightseconds, the powder is estimated to have been dropped as required.

(II) The same steps as above are repeated three times to see if thepowder is dropped as required. In case where the powder is droppedsatisfactorily twice or more, the flowability of the powder is estimatedto be “Good”. In case where the powder is never dropped, the flowabilityof the powder is estimated to be “Not good”. In case where in threeseries of the dropping test, the powder has been dropped only one time,the dropping test is further conducted twice, and when the two series ofthe dropping test are both satisfactory, the flowability is estimated tobe “Good”. In other cases, the flowability is estimated to be “Notgood”.

(III) With respect to the powder estimated to be “Good”, the upperhopper is charged with two cups of powder by using the same 30 cc cup,and the dropping test of the powder is conducted in the same manner asabove. When as a result, the flowability is estimated to be “Good”, thenumber of cups filled with the powder is increased successively and thedropping test is continued until the flowability is estimated to be “Notgood”. The dropping test is conducted up to eight cups at most. Thepowder having flowed out from the lower hopper in the previous droppingtest may be re-used.

(IV) The larger the amount of the PTFE powder is, the more difficult todrop.

The number of cups when the flowability is estimated to be “Not good” issubtracted by 1, and the obtained value is taken as “Flowability 1” ofthe powder.

Flowability 2

Flowability 2 is measured and evaluated in the same manner as inFlowability 1 except that the diameter of the outlet of the lower hopperis changed to 8 mm. Average particle size:

Standard sieves of 10, 20, 32, 48 and 60 meshes (inch mesh) are placedin that order from the top, and PTFE powder is put on the 10 mesh sieve.The sieves are vibrated to drop smaller particles downward through eachsieve in order. Then after the ratio of the PTFE powder remaining oneach sieve is obtained by %, accumulated percentages (ordinate) of eachremaining powder to the openings of each sieve (abscissa) are plotted onthe logarithmic probability paper, and those points are connected with aline. The particle size, the proportion of which is 50% on that line, isobtained and is regarded as an average particle size.

Particle Size Distribution

The particle size distribution is a proportion in weight of theparticles having a diameter 0.7 to 1.3 times the average particle sizeon the basis of the whole particles, and is calculated by multiplyingthe average particle size by 0.7 or 1.3. The obtained values are plottedon the accumulated weight percentage curve, and thus the weightpercentage is obtained.

Tensile Strength (Hereinafter May be Referred to as “TS”) and Elongation(Hereinafter May be Referred to as “EL”)

A die having an inner diameter of 100 mm is charged with 25 g of powder,and a pressure is applied gradually over about 30 seconds until thefinal pressure becomes about 500 kg/cm². Then that pressure is kept fortwo minutes to give a pre-molded article. The pre-molded article istaken out of the die mold and put in an electric oven being kept at 365°C. to be subjected to sintering for three hours. Then the sinteredarticle is punched with a JIS dumbbell No. 3 to give a sample. A stressat break and elongation of the sample are measured in accordance withJIS K 6891-5.8 by stretching at a stretching rate of 200 mm/min with anautograph having a gross weight of 500 kg.

EXAMPLE 1

80 Kg (dry basis) of a PTFE powder (POLYFLON TFE MOLDING POWDER M-12which is PTFE homopolymer available from DAIKIN INDUSTRIES, LTD.) havingan average particle size of 31 μm after pulverizing, 15 kg of glassfibers (average diameter: 12 μm, average fiber length: 80 μm) which hadbeen subjected to water-repellent-treatment previously with anaminosilane coupling agent and 5 kg of molybdenum disulfide powder(average particle size: 4 μm) were pre-mixed by using a 500-literHenschel mixer.

A 150-liter ribbon mixer (available from Fuji Paudal Co., Ltd.) wascharged with 40 kg of the mixture comprising PTFE powder, glass fiberand molybdenum disulfide and obtained by the above-mentioned pre-mixing,followed by adding thereto 14 kg of an aqueous solution of 1.25% byweight of ammonium perfluorooctanoate over five minutes under theconditions of a temperature of 30° C. and rotation of 35 rpm. Afterstirring under the same conditions for 10 minutes, the mixture waspassed to a flash mill (available from Fuji Paudal Co., Ltd.) rotatingat 850 rpm to give a granular PTFE powder 1 containing glass fibers andmolybdenum disulfide. The granular PTFE powder 1 had an average particlesize of 640 μm, an apparent density of 0.61 g/cc, a flowability of 0 anda particle size distribution shown in Table 1.

40 Kg of the granular PTFE powder 1 was put in a 130-liter cross rotarymixer with a stirring vessel, and a temperature thereof was maintainedat 56° C. by refluxing hot water in its jacket. Then the stirring wascarried out for 20 minutes at 20 rpm of autorotation speed and 12 rpm ofrevolution speed (a ratio of revolution/autorotation: 0.6) for shapingof the granular powder.

Physical properties of the obtained granular PTFE powder after theshaping and physical properties of a molded article obtained from thepowder are shown in Table 1.

Comparative Example 1

A 150-liter ribbon mixer (available from Fuji Paudal Co., Ltd.) wascharged with 40 kg of the granular PTFE powder 1, and stirring wascarried out at 35 rpm at 30° C. for 10 minutes.

Physical properties of the obtained (usual) granular PTFE powder forcomparison after the shaping and physical properties of a molded articleobtained from the powder are shown in Table 1.

Also FIG. 3 shows a particle size distribution of the granular PTFEpowder 1 before the shaping, the granular PTFE powder after the shapingwhich was obtained in Example 1, and the granular PTFE powder after theshaping which was obtained in Comparative Example 1.

TABLE 1 After shaping Before Cross rotary Ribbon mixer shaping mixer(Ex. 1) (Com. Ex. 1) Average particle size (μm) 640 640 820 Apparentdensity (g/cc) 0.61 0.78 0.78 Flowability 1 (times) 0 8 8 Flowability 2(times) 0 6 2 Particle size distribution (%) 10 mesh on 1 0 1.6 20 meshon 30.6 27.4 46.2 32 mesh on 32.6 38.2 40 48 mesh on 25.6 26.8 11.4 60mesh on 3.6 4 0.6 60 mesh pass 7.4 3.6 0.2 Physical properties of moldedarticle Tensile strength 239 236 205 (kg/cm) Tensile elongation (%) 330330 300

In the column of the particle size distribution of Table, 10 mesh on, 20mesh on, 32 mesh on, 48 mesh on and 60 mesh on indicate the percentagesof particles remaining on the 10 mesh, 20 mesh, 32 mesh, 48 mesh and 60mesh, respectively. And, 60 mesh pass represents the percentage of theparticles passed through the 60 mesh sieve.

As is evident from Table 1 and FIG. 3, according to the method ofshaping of the present invention, not only an apparent density can beincreased but also enhancement of a flowability which is a primaryobject of shaping of the present invention can be achieved withoutchanging an average particle size and a particle size distributionsubstantially.

EXAMPLE 2

80 Kg (dry basis) of a PTFE powder (POLYFLON TFE MOLDING POWDER M-12which is PTFE homopolymer available from DAIKIN INDUSTRIES, LTD.) havingan average particle size of 31 μm after pulverizing and 14.1 kg of pitchtype carbon fibers (average diameter: 14.5 μm, average fiber length: 90μm) were pre-mixed by using a 500-liter Henschel mixer.

A 150-liter ribbon mixer (available from Fuji Paudal Co., Ltd.) wascharged with 40 kg of the mixture comprising the PTFE powder and pitchtype carbon fiber and obtained by the above-mentioned pre-mixing,followed by adding thereto 14 kg of an aqueous solution of 1.25% byweight of ammonium perfluorooctanoate over five minutes under theconditions of a temperature of 30° C. and rotation of 35 rpm. Afterstirring under the same conditions for 10 minutes, the mixture waspassed to a flash mill (available from Fuji Paudal Co., Ltd.) rotatingat 900 rpm to give a granular PTFE powder 2 containing carbon fibers.The granular PTFE powder 2 had an average particle size of 750 μm, anapparent density of 0.58 g/cc, a flowability of 0 and a particle sizedistribution shown in Table 2.

40 Kg of the granular PTFE powder 2 was put in a 130-liter cross rotarymixer with a stirring vessel, and a temperature thereof was maintainedat 58° C. by refluxing hot water in its jacket. Then the stirring wascarried out for 20 minutes at 20 rpm of autorotation speed and 6 rpm ofrevolution speed (a ratio of revolution/autorotation: 0.3) for shapingof the granular powder.

Physical properties of the obtained granular PTFE powder after theshaping and physical properties of a molded article obtained from thepowder are shown in Table 2.

Comparative Example 2

A 150-liter ribbon mixer (available from Fuji Paudal Co., Ltd.) wascharged with 40 kg of the granular PTFE powder 2, and stirring wascarried out at 25 rpm at 30° C. for 20 minutes.

Physical properties of the obtained (usual) granular PTFE powder forcomparison after the shaping and physical properties of a molded articleobtained from the powder are shown in Table 2.

TABLE 2 After shaping Before Cross rotary Ribbon mixer shaping mixer(Ex. 2) (Com. Ex. 2) Average particle size (μm) 750 760 820 Apparentdensity (g/cc) 0.58 0.69 0.67 Flowability 1 (times) 0 8 6.5 Particlesize distribution (%) 10 mesh on 1 0 1.4 20 mesh on 41 42.1 45.6 32 meshon 34.4 36.8 40.4 48 mesh on 13.8 12.6 10.8 60 mesh on 2.8 3.2 0.8 60mesh pass 8.2 5.3 1 Physical properties of molded article Tensilestrength 206 217 202 (kg/cm) Tensile elongation (%) 260 270 260

As it is evident from Table 2, according to the method of shaping of thepresent invention, not only an apparent density can be increased butalso enhancement of a flowability which is a primary object of shapingof the present invention can be achieved without changing an averageparticle size and a particle size distribution substantially.

EXAMPLE 3

20 Kg (dry basis) of a PTFE powder (POLYFLON TFE MOLDING POWDER M-12which is PTFE homopolymer available from DAIKIN INDUSTRIES, LTD.) havingan average particle size of 31 μm after pulverizing, 1.25 kg of agraphite powder (average particle size: 25 μm) and 3.75 kg of a whollyaromatic polyester resin powder (ECONOL available from Sumitomo ChemicalIndustries, Ltd., average particle size: 30 μm) were pre-mixed by usinga 150-liter Henschel mixer.

A 150-liter ribbon mixer (available from Fuji Paudal Co., Ltd.) wascharged with 40 kg of the mixture comprising the PTFE powder, graphiteand ECONOL and obtained by the above-mentioned pre-mixing, followed byadding thereto 14 kg of an aqueous solution of 1.25% by weight ofammonium perfluorooctanoate over five minutes at 30° C. at 35 rpm. Afterstirring under the same conditions for 10 minutes, the mixture waspassed to a flash mill (available from Fuji Paudal Co., Ltd.) rotatingat 1,100 rpm to give a granular PTFE powder 3 containing graphite andECONOL. The granular PTFE powder 3 had an average particle size of 710μm, an apparent density of 0.58 g/cc, a flowability of 0 and a particlesize distribution shown in Table 3.

40 Kg of the granular PTFE powder 3 was put in a 130-liter cross rotarymixer with a stirring vessel, and a temperature thereof was maintainedat 56° C. by refluxing hot water in its jacket. Then the stirring wascarried out for 20 minutes at 20 rpm of autorotation speed and 6 rpm ofrevolution speed (a ratio of revolution/autorotation: 0.3) for shapingof the granular powder.

Physical properties of the obtained granular PTFE powder after theshaping and physical properties of a molded article obtained from thepowder are shown in Table 3.

Comparative Example 3

A 150-liter ribbon mixer (available from Fuji Paudal Co., Ltd.) wascharged with 40 kg of the granular PTFE powder 3, and stirring wascarried out at 30 rpm at 30° C. for 8 minutes.

Physical properties of the obtained (usual) granular PTFE powder forcomparison after the shaping and physical properties of a molded articleobtained from the powder are shown in Table 3.

TABLE 3 After shaping Before Cross rotary Ribbon mixer shaping mixer(Ex. 3) (Com. Ex. 3) Average particle size (μm) 710 720 820 Apparentdensity (g/cc) 0.58 0.68 0.63 Flowability 1 (times) 0 8 3.5 Particlesize distribution (%) 10 mesh on 0.4 0 3.2 20 mesh on 36.0 39.8 44.9 32mesh on 34.8 33.2 37.9 48 mesh on 22.0 22 10.0 60 mesh on 3.6 2.8 2.0 60mesh pass 3.0 2.2 0.0 Physical properties of molded article Tensilestrength 132 146 137 (kg/cm) Tensile elongation (%) 230 240 230

As it is evident from Table 3, according to the method of shaping of thepresent invention, not only an apparent density can be increased butalso enhancement of a flowability which is a primary object of shapingof the present invention can be achieved without changing an averageparticle size and a particle size distribution substantially.

EXAMPLE 4

9.9 Kg (dry basis) of a PTFE powder (POLYFLON TFE MOLDING POWDER M-111which is a modified PTFE prepared by copolymerizing a small amount ofperfluoro(propyl vinyl ether) and available from DAIKIN INDUSTRIES,LTD.) having an average particle size of 25 μm after pulverizing and 1.1kg of pitch type carbon fibers (average diameter: 12 μm, average fiberlength: 110 μm) were pre-mixed by using a 75-liter Henschel mixer.

A 200-liter granulation tank was charged with 130 liters ofion-exchanged water and a tank temperature was adjusted to 25° C. Thenthe granulation tank was charged with 40 kg of the mixture comprisingthe PTFE powder and pitch type carbon fiber and obtained by theabove-mentioned pre-mixing. With stirring at 400 rpm with cone blades, anonionic surfactant (Unisafe A-LE available from NOF Corporation) wasadded thereto in an amount of 0.100% by weight, and 2 to 3 minutesafter, 25.0 kg of dichloromethane was added thereto. Subsequently thestirring was continued at 400 rpm for five minutes to makedichloromethane compatible with the mixture of the PTFE powder and pitchtype carbon fiber. After that, a product in the granulation tank waspassed into a line mixer outside the tank to carry out externalcirculation for 10 minutes. The inside temperature of the tank washeated up to 38° C. over 30 minutes and was maintained at thattemperature for 15 minutes to distill off dichloromethane. During thatperiod of time, the number of rotations of cone blades was 400 rpm.After stopping of the stirring, a granulate was separated from water byusing a 150 mesh sieve to give a granular PTFE powder 4. The granularPTFE powder 4 had an average particle size of 480 μm, an apparentdensity of 0.74 g/cc, a flowability of 6 and a particle sizedistribution shown in Table 4.

40 Kg of the granular PTFE powder 4 was put in a 130-liter cross rotarymixer with a stirring vessel, and a temperature thereof was maintainedat 38° C. by refluxing hot water in its jacket. Then the stirring wascarried out for 95 minutes at 24 rpm of autorotation speed and 12 rpm ofrevolution speed (a ratio of revolution/autorotation: 0.5) for shapingof the granular powder.

Physical properties of the obtained granular PTFE powder after theshaping are shown in Table 4. Also the powder was sampled every fiveminutes after starting of the shaping, and an average particle size andparticle size distribution were measured. The results are shown in FIG.4.

TABLE 4 After shaping Before Cross rotary mixer shaping (Ex. 4) Averageparticle size (μm) 480 480 Apparent density (g/cc) 0.74 0.82 Flowability1 (times) 6 8 Particle size distribution (%)* 10 mesh on 0.0 0.0 20 meshon 1.8 2.2 32 mesh on 42.4 43.1 48 mesh on 50.4 50.1 60 mesh on 4.4 4.080 mesh on 0.8 0.6 80 mesh pass 0.2 0.0 *A particle size distributionwas determined by using a 80 mesh sieve additionally.

As shown in FIG. 4, an apparent density increases rapidly from thestarting of the shaping and in about 30 minutes to about 40 minutes,reaches the top. On the contrary, an average particle size does notchange substantially throughout the shaping step.

Industrial Applicability

According to the method of shaping of the present invention, not only anapparent density of the granular PTFE powder can be increased but alsoenhancement of a flowability which is a primary object of shaping of thepresent invention can be achieved without changing an average particlesize and a particle size distribution substantially.

What is claimed is:
 1. A method of shaping of a granularpolytetrafluoroethylene powder; characterized in that a granular PTFEpowder granulated to an average particle size of from 100 to 800 μm isput in a stirring vessel provided with two or more rotation axescrossing each other and is subjected to stirring for shaping by rotatingthe stirring vessel on its rotation axes.
 2. The method of shaping ofclaim 1, wherein the shaping is carried out under the conditions ofincreasing an apparent density of the granular polytetrafluoroethylenepowder without substantially changing its average particle size andparticle size distribution.
 3. The method of shaping of claim 2, whereina ratio of change in an average particle size of the granularpolytetrafluoroethylene powder after the shaping is within ±10%.
 4. Themethod of shaping of claim 1, wherein two rotation axes of the stirringvessel are disposed at a right angle to each other.
 5. The method ofshaping of claim 4, wherein the stirring vessel is a cross rotary mixer.6. The method of shaping of claim 5, wherein a speed of autorotation ofthe cross rotary mixer is from 10 to 30 rpm and a speed of revolutionthereof is from 5 to 20 rpm.
 7. The method of shaping of claim 3,wherein the stirring vessel has two rotation axes disposed at a rightangle to each other and the granular polytetrafluoroethylene powdercontains a filler.
 8. The method of shaping of claim 7, where in thegranular polytetrafluoroethylene power is a powder granulated by a drygranulation method.
 9. The method of shaping of claim 7, wherein thegranular polytetrafluoroethylene power is subjected to a deagglomerationbefore the shaping.
 10. The method of shaping of claim 8, wherein thegranular polytetrafluoroethylene power is subjected to deagglomerationbefore the shaping.
 11. The method of shaping of claim 3, wherein thestirring vessel has two rotation axes disposed at a right angle to eachother and the granular polytetrafluoroethylene powder does not contain afiller.
 12. The method of shaping of claim 11, wherein the granularpolytetrafluoroethylene power is a powder granulated by a drygranulation method.
 13. The method of shaping of claim 11, wherein thegranular polytetrafluoroethylene power is subjected to deagglomerationbefore the shaping.
 14. The method of shaping of claim 12, wherein thegranular polytetrafluoroethylene power is subjected to deagglomerationbefore the shaping.
 15. The method of shaping of claim 2, wherein thegranular polytetrafluoroethylene power is subjected to deagglomerationbefore the shaping.
 16. The method of shaping of claim 15, wherein thegranular polytetrafluoroethylene power is a powder granulated by a drygranulation method.
 17. The method of shaping of claim 2, wherein thegranular polytetrafluoroethylene power is a power granulated by a drygranulation method.